Image-converter tube with output fluorescent screen assembly resiliently mounted



Feb. 14, 1967 J. MESTA 3,304,455

IMAGE-CONVERTER TUBE WITH OUTPUT FLUORESCENT SCREEN ASSEMBLY RESILIENTLYMOUNTED Filed Jan. 6, 1964 2 Sheets$heet 1 //VV'/VTOR L J MestaATTORNEYS 1967 L. J. MESTA IMAGE-CONVERTER TUBE WITH OUTPUT FLUORESCENTSCREEN ASSEMBLY RESILIENTLY MOUNTED Filed Jan. 6, 1964 2 Sheets-Sheet 2ATTORNEYS United States Patent 3,304,455 IMAGE-CONVERTER TUBE WKTHOUTPUT FLUORESCENT SCREEN ASSEMBLY RESIL- IENTLY MOUNTED Lucien J.Mesta, Paris, France, assignor to Compagnie Francaise Thomson-Houston,Paris, France, a French body corporate Filed Jan. 6, 1964, fler. No.335,857 Claims priority, application France, Jan. 16, 1963, 921,573,Patent 1,388,626 7 Claims. (Cl. 313-94) This invention relates toelectron tubes of the eneral class including an output image-formingscreen, usually fluorescent or luminescent in character, provided in thesealed envelope of the tube, and upon which an image pattern is formedby impact of photo electrons on the screen in the operation of the tube.The image-forming screen may be adapted for direct visual observation orit may serve to transmit the image pattern formed on it to furtherapparatus. Tubes of this general class are herein termed image convertertubes.

One type of image converter tube to which the invention is moreespecially, though not exclusively directed is that of brightnessamplifier or image-intensifier tubes as used especially in X-ray work toincrease the brightness of a primary image produced by X-rays.

It is a general object of this invention to improve the construction ofimage converter tubes in respect to immunity of the output image fromdefects due to dust and the like contaminating particles present withinthe sealed envelope of the tube.

In the manufacture of large electron tubes there are many sources ofcontamination which no amount of care during fabrication is able toavoid. Minute particles and dust motes of all description floating aboutin the atmosphere of the shop, frequently small shreds of textile fabricfrom the operators clothing, become charged with electricity, find theirway into the tube before evacuation and cling to the tube walls byelectrostatic attraction, and finally become permanently sealed in thetube. Fragments of metal and metal oxides from electrode-solderingoperations performed in the tube are another source of contamination.Frequently, a photoemissive layer is formed within the tube aftersealing, through evaporation-deposition of alkali metals vaporized insitu from electrically heated evaporator receptacles enclosed in theenvelope. Such process leads to the presence of contaminant particleswithin the sealed tube, such as particles stripped off or/and evaporatedfrom the walls of the evaporator receptacles and chemical compoundsproduced by side reactions.

In brightness amplifier tubes of the kind used e.g. for intensifying anX-ray image, another and particularly troublesome source ofcontamination has been provided by the presence of the fluorescent layercoating the outer surface of the primary or input screen of the tube.The fluorescent coatings used in such tubes contain relatively fragileconstituents, and particles are apt to be stripped from them throughoutthe service life of the tube.

In view of the gain in brightness inherently provided by an imageconverter tube, and the optical magnification of its output image whenviewed through an optical instrument or other means of observation, thepresence of any such foreign particles that may have settled on orbeyond the surface of the output image screen of the tube is seen as aconspicuous flaw and detracts seriously from the utility of theapparatus.

For these reasons, certain delicate image converter tubes, such as imageorthicons used in television camera apparatus, have heretofore beenrequired to be manipulated carefully and operated only in a horizontalposi- 3,304,455 Patented Feb. 14, 1967 tion or at a small angle to thehorizontal plane, in order to avoid setting up a cloud of floatingparticles inside the tube which might then settle on and beyond theoutput screen and spoil the image. This is a serious limitation, and onethat is entirely unacceptable in many applications, such in particularas the X-ray image intensifier tubes earlier mentioned, since medicalX-ray work, for example must have to be carried out in any orientationof the tube, including the vertical.

It is an object of this invention to overcome the above difficulties andlimitations by positively preventing any foreign particles inevitablypresent in an image converter tube from being deposited on or beyond theoutput sur face of the output image forming screen of the tube. Anotherobject, relating more especially to image converter tubes provided witha fluorescent primary image-forming screen, is to prevent in a positivemanner any particles that may be stripped off the fluorescent surfacefrom entering the general tube cavity whence they may settle on orbeyond the output screen.

According to an important feature of the invention, there is provided inan image converter tube having a sealed envelope, a photocathode in theenvelope near an input end thereof, an output image-forming screenassembly in the envelope near to an output end thereof, andelectron-optical means in the envelope for directing photo electronsfrom the photocathode on to an input surface of said output screen toform an image pattern thereon, the improvement comprising an annulardust-shield member positioned generally in a space between said outputimage-forming screen assembly and the output end of the envelope, andhaving one marginal portion sealingly engaging the periphery of saidscreen assembly and another marginal portion sealingly engaging theinner surface of said envelope so as substantially to seal off the saidspace from the general cavity of the envelope.

According to another feature of the invention, more especiallyapplicable to X-ray image-intensifier tubes and the like, there isprovided in such a tube having a sealed envelope, a primary or inputscreen assembly in the envelope near an input end thereof said primaryscreen assembly including a supporting plate transparent to lightradiations, a fluorescent surface on its input side and a photoemissivesurface on its output side, a secondary or output image-forming screenin the envelope near the output end thereof and electron-optical meansfor directing photoelectrons from the photoemissive surface of theprimary screen on to an inner surface of the secondary screen to form animage pattern thereon, the improvement comprising means for sealing theprimary screen assembly comprising a pair of annular flanged supportingmembers having first flanges thereof sealed to peripheral portions ofopposite sides, respectively, of said primary screen assembly, andhaving second flanges projecting therefrom, and an annular sealing stripof generally U- shaped cross section having its leg respectivelysealingly secured to said second flanges whereby to seal off saidfluorescent surface of the primary screen assembly from the generalcavity of the envelope.

An exemplary embodiment of the invention will now be described forpurposes of illustration but not of limitation with reference to theaccompanying drawings, wherein:

FIG. 1 is a cross sectional view, with certain dimensions exaggeratedfor clarity, showing a primary or input screen assembly for an improvedbrightness intensifier tube;

FIG. 2 is a cross sectional view with certain dimensions exaggerated, ofthe output end of the same tube showing the secondary or output screenassembly provided with the dust shield member of the invention; and

FIG. 3 is a small-scale simplified view in section of the imageintensifier tube provided with both improvements shown in FIGS. 1 and 2,only the main components of the tube being shown.

The embodiment of the invention disclosed by way of example is animage-intensifier tube for X-rays. Referring first to FIG. 3, the tubecomprises a sealed evacuated glass envelope 27. Suitably supported inthe envelope near its input, herein upper, end is a primary screenassembly generally designated 28. This screen assembly, as will bedescribed in detail later with reference to FIG. 1, has a fluorescentcoating 2 on its input or upper side, and a photoemissive layer 3 on itsoutput or lower side. Mounted within the reduced output end, or lowerend, of the envelope 27 is a secondary or output screen assembly 32,which also will be described in detail later (FIG. 2), The screen has afluorescent coating 17 on its input or upper side.

A system of annular electron-optical electrodes are positioned coaxiallybetween the primary screen 28 and secondary screen 32. These electrodes,shown in outline at 29, 30, 31, and 40, are connected in operation tocertain definite potentials for focussing a beam of photoelectronsemitted from the photo-emissive under surface 3 of primary screen 28 onto the fluorescent upper surface 17 of secondary screen 32. A moredetailed description of the X-ray image intensifier tube here referredto may be found in the following copending patent applications: Prop.2946, Prop. 2954.

The general operation of the tube is as follows: X-rays striking thefluorescent upper surface 2 of primary or input screen 28 excitefluorescence in it and the resulting photons strike the photo-emissiveunder surface 3, or photocathode, of said primary screen assembly 28.Photoelectrons are thereby emitted in a pattern corresponding to theoriginal X-ray image. These photoelectrons are accelerated and at thesame time focussed by the action of the electrode system 29, 30, 31, 40,on to secondary screen 32. The photoelectrons excite the fluorescentupper surface 17 of secondary screen 32 and form an image correspondingaccurately to the original X-ray image but of reduced size and greatlyincreased brightness. When such image formed on screen 32 is viewedthrough a magnifying optical instrument to restore it to its initialdimensions, the gain in brightness or luminance is of the order ofseveral thousand times. Instead of direct viewing, the image may beexploited in other ways, e.g. photographically, cinematograp-hically ortelevisually.

It will be readily conceived that any small foreign particle penetratinginto the space between the secondary screen 32 and the end wall of thetube, will result in a serious flaw in the output image especially aftermagnification.

Such particles when settling on either the output side of the secondaryscreen assembly 32 or the inner surface of the end wall of glassenvelope 27 constituting a viewing window, will project highlycontrasting optical shadows which very seriously affect the quality ofthe observed image even when the particle originating the shadow isquite small. It will be readily understood that the settling ofparticles on the input side of the output screen assembly 32 is muchless troublesome because the incident beam of photoelectrons strikeseach point of the fluorescent layer 17 at a wide range of angles ofincidence, so that there will generally be an appreciable number ofelectrons to excite the fluorescent emission of the layer in the areaunder such a particle.

The sources of contaminating particles in the tube are many andunavoidable as earlier indicated. They include dust particles driftingin the shop atmosphere and which become electrostatically charged sothat they cannot be removed on evacuation of the tube. They also includefragments of metallic and oxide and other compounds generated duringevaporation deposition of the photocathode surface 3 of screen 28, afterthe tube has been scaled, from evaporator receptacles (not shown) whichmay be positioned within the specially formed central electrode 31 asdisclosed in a co-pending application. Perhaps the chief source ofcontamination within the tube is the fluorescent upper surface 2 of theprimary screen 28. The fluorescent coatings used in certain applicationscontain rather fragile constituents and tiny fragments tend to bestripped from it. These fragments, in addition to impairing the outputimage as described above, may also impair the photoemissive propertiesof the photocathode layer 3 on the under surface of the primary screen28.

Reference will now be made to FIG. 1 which shows in detail theconstruction of the primary screen assembly 28 according to theinvention. This assembly includes a thin domed glass plate 1 having theafore-mentioned photoemissive (photocathode) layer 3 provided on itsunder surface, preferably by the evaporation-deposition process withinthe sealed tube, earlier referred to. The upper surface of glass plate 1carries the fluorescent coating 2, of a suitable composition to beexcited by X-rays. The under surface of plate 1 is secured through aninterposed annular electrode 4 serving to apply an operating potentialto photocathode 3, to the upper surface of the inwardly projectingflange of an inner annular supporting member 6, of angle section, madeof suitable metal. The assembly further includes a thin sheet element 7of aluminum transparent to the incident X-rays, domed to the samecurvature as the glass plate 1 and spaced a short distance above theupper surface of fluorescent coating 2. The purpose of this aluminumshield is mainly to reflect back on to the primary screen a major amountof the photons emitted by the fluorescent layer 2 in an upwarddirection, and thereby increase the eificiency of the screen assembly.The periphery of aluminum sheet 7 is secured to the under surface of theinwardly projecting flange of an outer annular supporting member 5,which in turn is suitably supported from the inner wall surface of theenvelope of the tube, as by way of a Kovar seal or otherwise.

The annular supporting members 5 and 6 have downwardly projectingflanges 8 and 12 respectively, with the flange 12 of the inner memberbeing fitted into the flange 8 of the outer member. In this connectionit will be understood that in FIG. 1 the width of the gaps betweenvarious components has been exaggerated for clarity. Actually the innerflanged member 6 is fitted into the outer member 5 with relatively smallclearance. In practice however it is found that such clearance which maybe of the order of 0.2 mm. on the radius, is sufiicient to allow smallfragments from the less stable constituents of the X-ray sensitivefluorescent layer 2, to slip past the gap between the flanges 8, 12 ofthe supporting members.

In accordance with the invention, passage of such fragments through thegap between flanges 8 and 12 into the general cavity of the tube isprevented by the provision of a sealing strip member 10, of annular formand U-shaped cross section as shown, preferably made of thin aluminumfoil of e.g. 0.05 mm. thickness. The sealing strip 10 has its outer legbonded to the inner surface of the outer flange 8, and its inner legbonded to the inner surface of the inner flange 12, as shown. Thus theseal 10 acts to trap particles dislodged from the fluorescent coating 2and prevent their fouling the secondary screen 32 and/or thephotocathode surface 3.

Referring now to FIG. 2, the secondary screen assembly 32 comprises aflat glass plate 18 having a fluorescent layer 17 deposited on its uppersurface. The under surface of glass plate 18 is supported at itsperiphery on the inner marginal part of a flat, metallic annularsupporting member 20. Member 20 may be peripherally supported from theinner surface of tube envelope 27 through any suitable means, e.g., aKovar seal. A metallic retainer member 19 of the flanged cross sectionalshape shown has a lower flange secured to the outer periphery ofsupporting member 20 as by spots of solder 21, and its inwardlyprojecting upper flange overlies the outer edge part of the fluorescentlayer 17. An annular spring member 22 or spring washer is clampedbetween the upper surface of layer 17 and the under surfaces of saidupper flange of retainer 19, so as to maintain the secondary screen inresiliently assembled relation with its supporting means.

Soldered as at 24 to the under surface of flat annular supporting member20 around its inner periphery, is the inturned upper flange 25- of agenerally frustoconical, annular, metallic dust shield member 23. Thelarger base of this frustoconical member has an out-turned, roundedflange 26 which engages the upper surface of the end wall of the tubeenvelope 27. The dust shield member 23 positively protects the spacebetween the secondary screen assembly and the tube end wall against theingress of foreign particles. In the manufacture of the tube, it isconvenient that relatively large tolerance should be available as to theprecise point at which supporting member 20 is supported from the glasstube casing, and hence the free space between the secondary screen plate1 and the tube end wall may vary from one tube to another. Theresiliency of the dust shield member 23 due especially to the roundedlower-flange 25 makes it possible to take up such dimensionalvariations. The flared fructoconical shape of dust-shield member 23prevents unwanted reflection of light from the fluorescent coating 17 atthe inner surfaces of said member.

X-ray image intensifier tubes constructed as here disclosed andembodying simultaneously both main features of the invention, namely thesealing strip preventing egress of fluorescent particles from out of theprimary screen assembly, and dust shield 23 preventing ingress ofparticles from whatever source into the space under the secondary screenassembly 32, have given excellent performance in regard to the clearnessand general quality of the output images produced by them. The extremelytroublesome effects of floating dust particles sealed in the tube on thequality and resolution of all parts of the output image, a constantsource of trouble to users of tubes of this general class in the past,are completely eliminated, no matter how the tube is manipulated ororiented in use. It will be realized however that the two improvementsjust mentioned may well be used separately. In particular, inimage-converter tubes having no fluorescent layer provided on theirprimary screen assembly, it is selfevident that the sealing strip 10would be superfluous. Various other departures from the exemplaryembodiment illustrated and described may be conceived within the scopeof the invention.

I claim:

1. In an image converter tube having a sealed envelope, a photocathodein the envelope near an input end thereof, an image-forming outputscreen assembly in the envelope near and spaced from an output endthereof, and electronoptical means in the envelope for directingphotoelectrons from the photoacthode on to an inner surface of saidoutput screen to form an image pattern thereon, the improvementcomprising means supporting said output screen assembly at a prescribedspacing from said output end of the tube including spring meansresiliently pressing the output screen assembly against its supportingmeans an annular dust-shield member positioned in the space between theoutput screen assembly and the output end of the envelope and having onemarginal portion sealingly engaging the periphery of the screen assemblyand another marginal portion sealingly engaging the inner surface ofsaid envelope so as to seal off said space from the general cavity ofthe tube envelope against ingress of foreign particles thereinto.

2. In an image converter tube having a sealed envelope, a photocathodein the envelope near an input end thereof,

an image-forming'output screen assembly in the envelope near and spacedfrom an output end thereof, and electronoptical means in the envelopefor directing photoelectrons from the photocathode on to an innersurface of said output screen to form an image pattern thereon, theimprovement comprising a resilient, annular, f-rustoconical dustshieldmember positioned in the space between the output screen assembly andthe output end of the envelope and having its smaller base portionsealingly engaging the periphery of the screen assembly and its largerbase portion sealingly engaging the inner surface of the end wall ofsaid envelope so as to seal off said space from the general cavity ofthe tube envelope against ingress of foreign particles thereinto, saidresilient member being capable of compensating for variations in thedistance from the output screen assembly to the end wall of the tubeenvelope.

3. In an image converter tube having a sealed envelope, an inputimage-forming screen assembly in the envelope near an input end thereofincluding a fluorescent layer on an input side and a photoemissive layeron an output side of said input screen assembly, an out-put imageformingscreen assembly in the envelope near the output end thereof andelectron-optical means for directing photoelectrons from saidphotoemissive layer of the input screen assembly to an image-forminginput surface of the output screen assembly, the improvement comprisingmeans for sealing the input screen assembly comprising a pair of flangedannular supporting members having first flanges sealed to peripheralportions of opposite sides, respectively, of the input screen assemblyand having second flanges projecting therefrom, and an annular sealingstrip of generally U-shaped cross section having its legs respectivelysealingly engaging the second flanges of the respective members wherebyto seal off said fluorescent layer from the general cavity of the tubeenvelope against the egress of fluorescent particles into the cavity.

4. A tube according to claim 3 wherein the sealing strip comprisesaluminum foil.

5. A tube according to claim 3 including the further improvementcomprising an annular dust-shield member positioned in a space providedbetween the output screen assembly and the output end of the envelopeand having one marginal portion sealingly engaging the periphery of thescreen assembly and another marginal portion sealingly engaging theinner surface of said envelope so as to seal off said space from thegeneral cavity of the tube envelope against ingress of foreign particlesthereinto.

6. In an image converter tube having a sealed envelope, an inputimage-forming screen assembly in the envelope near an input end thereof,said assembly comprising, serially-disposed in the direction ofpropagation of input radiations, a sheet element transparent to suchradiations, a fluorescent layer and a photoemissive layer, an outputimage-forming screen assembly in the envelope near the output endthereof and electron-optical means for directing photoelectrons fromsaid photoemissive layer of the input screen assembly to animage-forming input surface of the output screen assembly, theimprovement comprising means for sealing the input screen assemblycomprising a pair of flanged annular supporting members having firstflanges sealed to peripheral portions of said sheet element and saidphoto-emissive layer respectively and having second flanges projectingtherefrom, and an annular seal ing strip of generally U-shaped crosssection having its legs respectively sealingly engaging the secondflanges of the respective members whereby to seal off said fluorescentlayer from the general cavity of the tube envelope against the egress offluorescent particles into the cavity.

7. A primary image-forming screen assembly for an image-intensifier tubecomprising in combination a supporting plate transparent to light, afluorescent coating on an input side surface of the plate, aphotoemissive coating on an output side surface of the plate, ashielding sheet element transparent to input radiations supportedadjacent the input side of said fluorescent coating, a first flanged,annular supporting member having a first flange sealingly secured to theperiphery of said shielding element and a second flange projectingtherefrom towards the output side of said screen assembly, a secondflanged, annular supporting member having a first flange sealinglysecured to the periphery of said photoemissive coating and a secondflange projecting therefrom towards the output side of said screenassembly and fitted a loose fit into the second flange of the firstannular supporting member, and an annular sealing strip of U-shapedcross section having an outer leg sealingly engaging a surface of thesecond flange of said first annular supporting member and an inner legsealingly engaging a surface of the second flange of said secondsupporting member whereby to seal off the interior of said screenassembly against the egress of particles stripped off said fluorescentcoating.

References Cited by the Examiner UNITED STATES PATENTS 2,418,780 4/1947Leverenz 3l392 X 2,640,162 5/1953 Espenschied et al. 313--65 X 3,026,4373/1962 Niklas 313-65 10 3,043,974 7/1962 McGee 313-65 JAMES W. LAWRENCE,Primary Examiner.

ROBERT SEGAL, Examiner.

1. IN AN IMAGE CONVERTER TUBE HAVING A SEALED ENVELOPE, A PHOTOCATHODEIN THE ENVELOPE NEAR AN INPUT END THEREOF, AN IMAGE-FORMING OUTPUTSCREEN ASSEMBLY IN THE ENVELOPE NEAR AND SPACED FROM AN OUTPUT ENDTHEREOF, AND ELECTRONOPTICAL MEANS IN THE ENVELOPE FOR DIRECTINGPHOTO-ELECTRONS FROM THE PHOTOCATHODE ON TO AN INNER SURFACE OF SAIDOUTPUT SCREEN TO FORM AN IMAGE PATTERN THEREON, THE IMPROVEMENTCOMPRISING MEANS SUPPORTING SAID OUTPUT SCREEN ASSEMBLY AT A PRESCRIBEDSPACING FROM SAID OUTPUT END OF THE TUBE INCLUDING SPRING MEANSRESILIENTLY PRESSING THE OUTPUT SCREEN ASSEMBLY AGAINST ITS SUPPORTINGMEANS AN ANNULAR DUST-SHIELD MEMBER POSITIONED IN THE SPACE BETWEEN THEOUTPUT SCREEN ASSEMBLY AND THE OUTPUT END OF THE ENVELOPE AND HAVING ONEMARGINAL PORTION SEALINGLY ENGAGING THE PERIPHERY OF THE SCREEN ASSEMBLYAND ANOTHER MARGINAL PORTION SEALINGLY ENGAGING THE INNER SURFACE OFSAID ENVELOPE SO AS TO SEAL OFF SAID SPACE FROM THE GENERAL CAVITY OFTHE TUBE ENVELOPE AGAINST INGRESS OF FOREIGN PARTICLES THEREINTO.